My concern with the Jetstream would not be the main frame hinge, it would actually be the folding stem hinge. Off-road riding that truly requires an off-road bike often involves pulling on the bars, standing, aggressively manoeuvring, things which are not recommend for folding stems are commonplace. I would be worried about the longevity and reliability of that hinge if you were really doing off-road work worthy of a bike made for the job. I see that the folding hinge is nearly identical to one that would be on a more mundane urban Dahon, with the exception that the stem is not quite as long as the Jetstream, which would allow it a little extra resilience. Probably fine for the relatively tame trails such as the ones shown in the photographs.

I think the Rhino, Pocket Gnu and Pocket Llama are the 3 most genuine off-road folders you're going to see, and are full, front, and no suspension respectively.